Semispecular hollow backlight with gradient extraction

ABSTRACT

A backlight that includes a front reflector and a back reflector that form a hollow light recycling cavity including an output surface is disclosed. The backlight also includes a semi-specular element, and a light extraction element disposed within the hollow cavity. The light extraction element has a gradient specularity. The backlight also includes one or more light sources disposed to inject light into the hollow light recycling cavity, where the one or more light sources are configured to inject light over a limited angular range.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. 371 ofPCT/US2009/060308, filed Oct. 12, 2009, which claims priority to U.S.Application No. 61/108,606, filed Oct. 27, 2008, the disclosure of whichis incorporated by reference in their entirety herein.

BACKGROUND

Backlights can be considered to fall into one of two categoriesdepending on where the light sources are positioned relative to theoutput area of the backlight, where the backlight “output area”corresponds to the viewable area or region of the display device. The“output area” of a backlight is sometimes referred to herein as an“output region” or “output surface” to distinguish between the region orsurface itself and the area (the numerical quantity having units ofsquare meters, square millimeters, square inches, or the like) of thatregion or surface.

The first category is “edge-lit.” In an edge-lit backlight, one or morelight sources are disposed—from a plan-view perspective—along an outerborder or periphery of the backlight construction, generally outside thearea or zone corresponding to the output area. Often, the lightsource(s) are shielded from view by a frame or bezel that borders theoutput area of the backlight. The light source(s) typically emit lightinto a component referred to as a “light guide,” particularly in caseswhere a very thin profile backlight is desired, as in laptop computerdisplays. The light guide is a clear, solid, and relatively thin platewhose length and width dimensions are on the order of the backlightoutput area. The light guide uses total internal reflection (TIR) totransport or guide light from the edge-mounted lamps across the entirelength or width of the light guide to the opposite edge of thebacklight, and a non-uniform pattern of localized extraction structuresis provided on a surface of the light guide to redirect some of thisguided light out of the light guide toward the output area of thebacklight. Such backlights typically also include light managementfilms, such as a reflective material disposed behind or below the lightguide, and a reflective polarizing film and prismatic BEF film(s)disposed in front of or above the light guide, to increase on-axisbrightness.

In the view of Applicants, drawbacks or limitations of existing edge-litbacklights include the following: the relatively large mass or weightassociated with the light guide, particularly for larger backlightsizes; the need to use components that are non-interchangeable from onebacklight to another, since light guides must be injection molded orotherwise fabricated for a specific backlight size and for a specificsource configuration; the need to use components that requiresubstantial spatial non-uniformities from one position in the backlightto another, as with existing extraction structure patterns; and, asbacklight sizes increase, increased difficulty in providing adequateillumination due to limited space or “real estate” along the edge of thedisplay, since the ratio of the perimeter to the area of a rectangledecreases linearly (1/L) with the characteristic in-plane dimension L(e.g., length, or width, or diagonal measure of the output region of thebacklight, for a given aspect ratio rectangle).

The second category of backlight is “direct-lit.” In a direct-litbacklight, one or more light sources are disposed—from a plan-viewperspective—substantially within the area or zone corresponding to theoutput area, normally in a regular array or pattern within the zone.Alternatively, one can say that the light source(s) in a direct-litbacklight are disposed directly behind the output area of the backlight.A strongly diffusing plate is typically mounted above the light sourcesto spread light over the output area. Again, light management films,such as a reflective polarizer film, and prismatic BEF film(s), can alsobe placed above the diffuser plate for improved on-axis brightness andefficiency.

In the view of Applicants, drawbacks or limitations of existingdirect-lit backlights include the following: inefficiencies associatedwith the strongly diffusing plate; in the case of LED sources, the needfor large numbers of such sources for adequate uniformity andbrightness, with associated high component cost and heat generation; andlimitations on achievable thinness of the backlight beyond which lightsources produce non-uniform and undesirable “punchthrough,” wherein abright spot appears in the output area above each source.

In some cases, a direct-lit backlight may also include one or some lightsources at the periphery of the backlight, or an edge-lit backlight mayinclude one or some light sources directly behind the output area. Insuch cases, the backlight is considered “direct-lit” if most of thelight originates from directly behind the output area of the backlight,and “edge-lit” if most of the light originates from the periphery of theoutput area of the backlight.

Backlights of one type or another are usually used with liquid crystal(LC)-based displays. Liquid crystal display (LCD) panels, because oftheir method of operation, utilize only one polarization state of light,and hence for LCD applications it may be important to know thebacklight's brightness and uniformity for light of the correct oruseable polarization state, rather than simply the brightness anduniformity of light that may be unpolarized. In that regard, with allother factors being equal, a backlight that emits light predominantly orexclusively in the useable polarization state is more efficient in anLCD application than a backlight that emits unpolarized light.Nevertheless, backlights that emit light that is not exclusively in theuseable polarization state, even to the extent of emitting randomlypolarized light, are still fully useable in LCD applications, since thenon-useable polarization state can be eliminated by an absorbingpolarizer provided between the LCD panel and the backlight.

SUMMARY

In one aspect, the present disclosure provides a backlight that includesa partially transmissive front reflector and a back reflector that forma hollow light recycling cavity having an output surface. The backlightalso includes a semi-specular element and a light extraction elementdisposed within the hollow light recycling cavity. The light extractionelement has a gradient specularity. The backlight further includes atleast one light source disposed to inject light into the hollow lightrecycling cavity. The at least one light source is configured to injectlight over a limited angular range.

In another aspect, the present disclosure provides a display system thatincludes a display panel, and a backlight disposed to provide light tothe display panel. The backlight includes a partially transmissive frontreflector and a back reflector that form a hollow light recycling cavityhaving an output surface. The backlight also includes a semi-specularelement and a light extraction element disposed within the hollow lightrecycling cavity. The light extraction element has a gradientspecularity. The backlight further includes at least one light sourcedisposed to inject light into the hollow light recycling cavity. The atleast one light source is configured to inject light over a limitedangular range.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification reference is made to the appended drawings,where like reference numerals designate like elements, and wherein:

FIG. 1 is a schematic cross-section view of one embodiment of anedge-lit hollow backlight.

FIG. 2 is a schematic cross-section view of another embodiment of anedge-lit hollow backlight.

FIG. 3 is a schematic cross-section view of one embodiment of adirect-lit hollow backlight.

FIGS. 4A-4C are schematic cross-section views of various embodiments offront reflectors.

FIGS. 5 a-5 f are schematic cross-section views of various embodimentsof edge-lit hollow backlights.

FIG. 6 is a schematic cross-section view of one embodiment of a displaysystem.

FIG. 7 is a graph of modeled output light flux versus position.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like or similar components. However, it will beunderstood that the use of a number to refer to a component in a givenfigure is not intended to limit the component in another figure labeledwith the same number.

DETAILED DESCRIPTION

In general, the present disclosure describes several embodiments ofthin, hollow backlights that can be configured to provide selectedoutput light flux distributions. For example, in some embodiments, thebacklights of the present disclosure can be configured to provide auniform light flux distribution at output surfaces of the backlights.The term “uniform” refers to light flux distributions that have noobservable brightness features or discontinuities that would beobjectionable to a viewer. The acceptable uniformity of an output lightflux distribution will often depend on the application, e.g., a uniformoutput light flux distribution in a general lighting application may notbe considered uniform in a display application.

Further, for example, at least one or more of the embodiments ofbacklights of the present disclosure can be configured to provide anoutput light flux distribution that has an increased flux near a centerregion of the output surface compared to the flux near an edge region ofthe backlight. In some embodiments, a ratio of the luminance proximate acenter region of the output surface to the luminance proximate an edgeregion of the output surface is at least about 1.10. While such anoutput light flux distribution may be considered non-uniform, for someapplications, this type of distribution may be desired. Any suitableoutput light flux distribution can be provided.

In at least one of these embodiments, the backlight includes a partiallytransmissive front reflector and a back reflector that form a hollowlight recycling cavity having an output surface. The backlight furtherincludes a light extraction element disposed in the light recyclingcavity, and the light extraction element has a gradient specularity.This exemplary backlight can also include at least one semi-specularelement disposed within the cavity, and one or more light sourcesdisposed to emit light into the cavity over a limited angular range.

As used herein, the term “output light flux distribution” refers to thevariation in brightness over the output surface of the backlight. Theterm “brightness” refers to the light output per unit area into a unitsolid angle (cd/m²).

While not wishing to be bound by any particular theory, the output lightflux distribution of the backlights described herein can be tailored bycontrolling one or more of the following parameters:

1. The positioning of the front reflector relative to the backreflector;

2. The shape of one or both of the front and back reflectors;

3. The reflective and transmissive properties of the front and backreflectors;

4. The reflective, transmissive, and diffusive properties of the lightextraction element;

5. The reflective properties of the at least one semi-specular element;and

6. The average flux deviation angle of light emitted into the cavity bythe one or more light sources.

Controlling these factors involves balancing between filling the lightrecycling cavity with light and turning or redirecting light within thecavity such that at least a portion transmits through the frontreflector at desired locations of the output surface.

In general, light propagating within the cavity can be thought of asfalling into two angular distributions or zones: the transport zone andthe transmission zone. The transport zone includes light propagating indirections within the cavity such that the light is not likely to betransmitted through the front reflector. The angular range of light inthe transport zone will at least in part depend on the reflection andtransmission properties of the front and back reflectors; thereflection, transmission, and diffusion properties of the lightextraction element; the reflection properties of the semi-specularelement within the cavity; and the geometry of the cavity. For example,in some embodiments of backlights described herein, various frontreflectors are included that exhibit an increasing reflectivity forlight that is incident, e.g., within an angle with the front reflector'smajor surface of 30 degrees or less. For these front reflectors, thetransport zone can be defined as including light that is propagatingwithin the cavity in a direction that is within 30 degrees of thesurface of the front reflector. In other embodiments, the frontreflector may not exhibit this increased reflectivity for off-anglelight. In these embodiments, the transport zone may be defined asincluding light propagating within the cavity in a direction that issubstantially parallel to a major surface of the front reflector.

The transmission zone includes light propagating within the cavity indirections that allow at least a portion of such light to transmitthrough the front reflector. In other words, the transmission zoneincludes propagating light that is not in the transport zone.

For a backlight having a front reflector and back reflector that aresubstantially parallel, a light flux distribution at an output surfaceof the backlight can at least in part be determined by the rate ofconversion of light from the transport zone to the transmission zone.This rate depends on several factors, e.g., the reflectance andspecularity of the front, back, and edge reflectors of the backlight,the number of illuminated edges of the backlight, the light injectionangle of the one or more light sources, and the ratio of the length L ofthe backlight to the thickness H. By adjusting H, the rate of conversionof light from the transport zone to the transmission zone can at leastin part be controlled.

The thickness H can be adjusted by positioning the front and backreflectors such that at least a portion of the back reflector isnon-parallel to the front reflector. For example, as is furtherdescribed herein, the back reflector can be positioned to form awedge-shaped hollow light recycling cavity with the front reflector.This wedge shape provides an H that varies in at least one direction inthe hollow light recycling cavity.

The thickness H of the light recycling cavity can also be adjusted byshaping one or both of the front and back reflectors to be non-planar.As used herein, the term “non-planar” refers to a reflector, eitherfront or back, that cannot be substantially contained within a plane. Areflector having sub-millimeter structures formed on a substantiallyplanar substrate would not be considered non-planar for purposes of thisapplication. In some embodiments, a backlight can include a non-planarback reflector that includes one or more portions that slope towards thefront reflector. These sloping portions can be positioned to provide anincreased conversion rate of light from the transport zone to thetransmission zone at desired locations within the cavity. Backlightshaving non-planar back reflectors are further described in, for example,U.S. Patent Application No. 61/030,767 entitled BACKLIGHTS HAVINGSELECTED OUTPUT LIGHT FLUX DISTRIBUTIONS AND DISPLAY SYSTEMS USING SAME.

The light flux distribution produced by one or more of the backlightsdescribed herein can also be controlled in part by selecting thereflective and/or transmissive properties of one or both of the frontand back reflectors. For example, in the case of a backlight designed toemit only light in a particular (useable) polarization state, the frontreflector can have a high reflectivity for such useable light to supportlateral transport or spreading, and for light ray angle randomization toachieve acceptable spatial uniformity of the backlight output, but ahigh enough transmission into the appropriate application-useable anglesto ensure that application brightness of the backlight is acceptable.Further, in some embodiments, the front reflector of the recyclingcavity has a reflectivity that generally increases with angle ofincidence, and a transmission that generally decreases with angle ofincidence, where the reflectivity and transmission are for unpolarizedvisible light and for any plane of incidence, and/or for light of auseable polarization state incident in a plane for which oblique lightof the useable polarization state is p-polarized (and further, the frontreflector has a high value of hemispheric reflectivity while also havinga sufficiently high transmission of application-useable light).

The light flux distribution produced by one or more of the backlightsdescribed herein can also be controlled in part by selecting thereflective, transmissive, and diffusive properties of the lightextraction element. The light extraction element has a gradientspecularity, i.e. the path the light ray takes after interaction withthe light extraction element depends where the light ray intersects thelight extraction element. The light extraction element can include lightextraction patterns that are positioned precisely to extract light byrefraction, reflection, diffusion or similar processes. In oneembodiment, the light extraction elements can include particulates suchas refractive or diffusive beads, diffusing particles, down-convertingmaterials such as phosphors, microstructures, textures and the like.Examples of light extraction elements can be found, for example, in U.S.Pat. No. 6,845,212 (Gardiner et al.) and U.S. Pat. No. 7,223,005 (Lambet al.); and also in U.S. patent application Ser. No. 11/421,241.Examples of phosphors include those suitable for LED convertingmaterials, described elsewhere. The extraction features can be grooves,lenslets, or other microstructured or printed features designed toextract light from the backlight. The extraction features can beimparted to the lightguide using several methods, including but notlimited to: casting, embossing, microreplicating, printing, ablating,etching and other methods known in the art.

An exemplary embodiment of backlight also includes at least onesemi-specular element, the reflective properties of which can also beselected to in part determine the output light flux distribution. Forexample, the semi-specular element or elements can provide the hollowlight recycling cavity with a balance of specular and diffusecharacteristics, the elements having sufficient specularity to supportsignificant lateral light transport or mixing within the cavity, butalso having sufficient diffusivity to substantially homogenize theangular distribution of steady state light within the cavity, even wheninjecting light into the cavity only over a narrow range of angles (andfurther, in the case of a backlight designed to emit only light in aparticular (useable) polarization state, recycling within the cavitypreferably includes a degree of randomization of reflected lightpolarization relative to the incident light polarization state, whichallows a mechanism by which non-useable polarized light is convertedinto useable polarized light). In some embodiments, the light extractionelement and the semi-specular element can be combined into one opticalelement disposed in the cavity; however, they can also be separateelements disposed in the cavity.

Finally, the average flux deviation angle of light emitted into thecavity by the one or more light sources can be controlled to helpprovide the desired collimation of light injected into the cavity. Forexample, the backlights described herein can include light injectionoptics that partially collimate or confine light initially injected intothe recycling cavity to propagation directions close to a transverseplane (the transverse plane being parallel to the output area or surfaceof the backlight), e.g., an injection beam having an average fluxdeviation angle from the transverse plane in a range from 0 to 40degrees, or 0 to 30 degrees, or 0 to 15 degrees. In addition to the fluxdeviation angle, the shape of the light emitted into the cavity by thelight sources can also be controlled. For example, the emitted light canbe radially symmetrical about an emission axis.

Backlights for LCD panels, in their simplest form, consist of lightgeneration surfaces such as the active emitting surfaces of LED dies orthe outer layers of phosphor in a CCFL bulb, and a geometric and opticalarrangement of distributing or spreading this light in such a way as toproduce an extended- or large-area illumination surface or region,referred to as the backlight output surface. Generally, this process oftransforming very high brightness local sources of light into alarge-area output surface results in a loss of light because ofinteractions with all of the backlight cavity surfaces, and interactionswith the light-generation surfaces. To a first approximation, any lightthat is not delivered by this process through the output area or surfaceassociated with a front reflector—optionally into a desired applicationviewer-cone (if any), and with a particular (e.g., LCD-useable)polarization state (if any)—is “lost” light. In a commonly assignedrelated application, we describe a methodology of uniquelycharacterizing any backlight containing a recycling cavity by twoessential parameters. See U.S. Patent Application No. 60/939,084,entitled THIN HOLLOW BACKLIGHTS WITH BENEFICIAL DESIGN CHARACTERISTICS.

A backlight cavity, or more generally any lighting cavity, that convertsline or point sources of light into uniform extended area sources oflight can be made using a combination of reflective and transmissiveoptical components. In many cases, the desired cavity is very thincompared to its lateral dimension.

Historically, solid light guides have generally been used for thethinnest backlights and, except for very small displays such as thoseused in handheld devices, have been illuminated with linearly continuouslight sources such as cold cathode fluorescent lights (CCFLs). A solidlight guide provides low loss transport of light and specularreflections at the top and bottom surfaces of the light guide via thephenomenon of total internal reflection (TIR) of light. The specularreflection of light provides the most efficient lateral transport oflight within a light guide. Extractors placed on the top or bottomsurface of a solid light guide redirect the light to direct it out ofthe light guide, creating in essence a partial reflector.

Solid light guides, however, present several problems for large displayssuch as cost, weight, and light uniformity. The problem with uniformityfor large area displays has increased with the advent of separatered/green/blue (RGB) colored LEDs, which are effectively point sourcesof light compared to the much larger area of the output region of thebacklight. The high intensity point sources can cause uniformityproblems with conventional direct-lit backlights as well as edge-litsystems that utilize solid light guides. The uniformity problems can begreatly reduced if a hollow light guide could be made that also providesfor significant lateral transport of light as in a solid light guide. Insome cases for polarization and light ray angle recycling systems, ahollow cavity can be more proficient at spreading light laterally acrossa display face than a solid cavity. Some of the components that can beused to accomplish this effectively for a hollow light guide have notgenerally been available to the backlight industry, or in cases wherethe components already existed, the hollow light guides have not untilnow been constructed in the correct fashion to make a uniform, thin,efficient hollow light mixing cavity.

An efficient hollow reflective cavity has several advantages over asolid light guide for making a thin uniform backlight, even though asolid light guide does provide efficient top and bottom reflectors viathe phenomenon of Total Internal Reflection (TIR). The solid light guideis used primarily to provide a lateral dispersion of the light beforethe light interacts with other components such as reflective polarizersand other brightness enhancement films.

However, the TIR surfaces of a solid guide are inadequate to meet allthe needs of modern backlights, and additional light control films aretypically added both above and below the solid light guide. Most systemsthat use a solid light guide today also use a separate back reflector toutilize brightness enhancement films such as BEF and DBEF (bothavailable from 3M Company, St. Paul, Minn.). These films recycle lightthat is extracted from the light guide but is unusable for the displaybecause of unsuitable polarization or angle of propagation. The backreflector is typically a white reflector, which is substantiallyLambertian in its reflection characteristics. However, much of thelateral transport is first achieved with the TIR surfaces of the solidguide, and the recycled light is converted and returned to the displaywith the Lambertian back reflector. If separate top and bottom lightmanagement films are required anyway, it can be more efficient to usethem alone to create a hollow light guide and also to simultaneouslyprovide the functions of a reflective polarizer and other brightnessenhancement films. In this manner, the solid guide, as well as otherbrightness enhancement films, can be omitted.

We propose replacing the solid light guide with air, and the TIRsurfaces of a solid light guide with high efficiency low-lossreflectors. These types of reflectors can be important for facilitatingoptimal lateral transport of the light within the backlight cavity.Lateral transport of light can be initiated by the optical configurationof the light source, or it can be induced by an extensive recycling oflight rays in a cavity that utilizes low loss reflectors.

We can replace the TIR surfaces of the solid light guide with spatiallyseparated low loss reflectors that fall into two general categories. Oneis a partially transmissive or partial reflector for the front face andthe second is a full reflector for the back and side faces. As describedabove, the latter are often added to solid light guide systems anyway.For optimal transport of light and mixing of light in the cavity, boththe front and back reflectors may be specular or semi-specular insteadof Lambertian. A semi-specular component of some type is usefulsomewhere within the cavity to promote uniform mixing of the light. Theuse of air as the main medium for lateral transport of light in largelight guides enables the design of lighter, lower cost, and more uniformdisplay backlights.

For a hollow light guide to significantly promote the lateral spreadingof light, the means of light injection into the cavity is important,just as it is in solid light guides. The format of a hollow light guideallows for more options for injecting light at various points in adirect lit backlight, especially in backlights with multiple butoptically isolated zones. In a hollow light guide system, the functionof the TIR and Lambertian reflectors can be accomplished with thecombination of a specular reflector and a semi-specular, forwardscattering diffusion element.

Exemplary partial reflectors (front reflectors) we describe here—forexample, the asymmetric reflective films (ARFs) described in co-ownedU.S. Application No. 60/939,079—provide for low loss reflections andalso for better control of transmission and reflection of polarizedlight than is possible with TIR in a solid light guide alone. Thus, inaddition to improved light distribution in a lateral sense across theface of the display, the hollow light guide can also provide forimproved polarization control for large systems. Significant control oftransmission with angle of incidence is also possible with the ARFsmentioned above. In this manner, light from the mixing cavity can becollimated to a significant degree as well as providing for a polarizedlight output with a single film construction.

In some embodiments, preferred front reflectors have a relatively highoverall reflectivity, to support relatively high recycling within thecavity. We characterize this in terms of “hemispheric reflectivity,”meaning the total reflectivity of a component (whether a surface, film,or collection of films) when light is incident on it from all possibledirections. Thus, the component is illuminated with light incident fromall directions (and all polarization states, unless otherwise specified)within a hemisphere centered about a normal direction, and all lightreflected into that same hemisphere is collected. The ratio of the totalflux of the reflected light to the total flux of the incident lightyields the hemispheric reflectivity, R_(hemi). Characterizing areflector in terms of its R_(hemi) is especially convenient forrecycling cavities because light is generally incident on the internalsurfaces of the cavity—whether the front reflector, back reflector, orside reflectors—at all angles. Further, unlike the reflectivity fornormal incidence, R_(hemi) is insensitive to, and already takes intoaccount, the variability of reflectivity with incidence angle, which maybe very significant for some components (e.g., prismatic films).

Further, in some embodiments, preferred front reflectors exhibit a(direction-specific) reflectivity that increases with incidence angleaway from the normal (and a transmission that generally decreases withangle of incidence), at least for light incident in one plane. Suchreflective properties cause the light to be preferentially transmittedout of the front reflector at angles closer to the normal, i.e., closerto the viewing axis of the backlight. This helps to increase theperceived brightness of the display at viewing angles that are importantin the display industry (at the expense of lower perceived brightness athigher viewing angles, which are usually less important). We say thatthe increasing reflectivity with angle behavior is “at least for lightincident in one plane,” because sometimes a narrow viewing angle isdesired for only one viewing plane, and a wider viewing angle is desiredin the orthogonal plane. An example is some LCD TV applications, where awide viewing angle is desired for viewing in the horizontal plane, but anarrower viewing angle is specified for the vertical plane. In othercases narrow angle viewing is desirable in both orthogonal planes so asto maximize on-axis brightness.

With this in mind, let us consider the meaning of specifying (if wedesire) that the front reflector “exhibit a reflectivity that generallyincreases with angle of incidence,” in the case where the frontreflector is an ARF such as is described in U.S. Patent Application No.60/939,079. The ARF includes a multilayer construction (e.g., coextrudedpolymer microlayers that have been oriented under suitable conditions toproduce desired refractive index relationships, and desired reflectivitycharacteristics) having a very high reflectivity for normally incidentlight in the block polarization state and a lower but still substantialreflectivity (e.g., 25 to 90%) for normally incident light in the passpolarization state. The very high reflectivity of block-state lightgenerally remains very high for all incidence angles. The moreinteresting behavior is for the pass-state light, since that exhibits anintermediate reflectivity at normal incidence. Oblique pass-state lightin the plane of incidence will exhibit an increasing reflectivity withincreasing incidence angle, due to the nature of s-polarized lightreflectivity (the relative amount of increase, however, will depend onthe initial value of pass-state reflectivity at normal incidence). Thus,light emitted from the ARF in a viewing plane will be partiallycollimated or confined in angle. Oblique pass-state light in the otherplane of incidence, however, can exhibit any of three behaviorsdepending on the magnitude and polarity of the z-axis refractive indexdifference between microlayers relative to the in-plane refractive indexdifferences, as discussed in the 60/939,079 application.

In one case, a Brewster angle exists, and the reflectivity of this lightdecreases with increasing incidence angle. This produces bright off-axislobes in a viewing plane parallel to the output surface, which areusually undesirable in LCD viewing applications (although in otherapplications this behavior may be acceptable, and even in the case ofLCD viewing applications this lobed output may be re-directed towardsthe viewing axis with the use of a prismatic turning film).

In another case, a Brewster angle does not exist or is very large, andthe reflectivity of the p-polarized light is relatively constant withincreasing incidence angle. This produces a relatively wide viewingangle in the referenced viewing plane.

In the third case, no Brewster angle exists, and the reflectivity of thep-polarized light increases significantly with incidence angle. This canproduce a relatively narrow viewing angle in the referenced viewingplane, where the degree of collimation is tailored at least in part bycontrolling the magnitude of the z-axis refractive index differencebetween microlayers in the ARF.

Of course, the reflective surface need not have asymmetric on-axispolarizing properties as with ARF. Symmetric multilayer reflectors, forexample, can be designed to have a high reflectivity but withsubstantial transmission by appropriate choice of the number ofmicrolayers, layer thickness profile, refractive indices, and so forth.In such a case the s-polarized components will increase with incidenceangle, in the same manner with each other. Again, this is due to thenature of s-polarized light reflectivity, but the relative amount ofincrease will depend on the initial value of the normal incidencereflectivity. The p-polarized components will have the same angularbehavior as each other, but this behavior can be controlled to be any ofthe three cases mentioned above by controlling the magnitude andpolarity of the z-axis refractive index difference between microlayersrelative to the in-plane refractive index differences.

Thus, we see that the increase in reflectivity with incidence angle (ifpresent) in the front reflector can refer to light of a useablepolarization state incident in a plane for which oblique light of theuseable polarization state is p-polarized. Alternately, such increase inreflectivity can refer to the average reflectivity of unpolarized light,in any plane of incidence.

In some embodiments, the back reflectors also have a high hemisphericalreflectivity for visible light, typically, much higher than the frontreflector since the front reflector is deliberately designed to bepartially transmissive in order to provide the required light output ofthe backlight. The hemispherical reflectivity of the back reflector isreferred to as R^(b) _(hemi), while that of the front reflector isreferred to as R^(f) _(hemi). Preferably, the product R^(f)_(hemi)*R^(b) _(hemi) is at least 45%.

FIG. 1 is a schematic cross-section view of one embodiment of abacklight 100. The backlight 100 includes a partially transmissive frontreflector 110 and a back reflector 120 that form a hollow lightrecycling cavity 130. The cavity 130 includes an output surface 135. Asis further described herein, cavity 130 further includes a lightextraction element 140 having a gradient specularity. The backlight 100also includes a semi specular element 150 disposed within the hollowlight recycling cavity 130, as is further described herein. In FIG. 1,light extraction element 140 is shown disposed adjacent a major surface122 of back reflector 120, and semi-specular element 150 is showndisposed adjacent a major surface 112 of partially transmissive frontreflector 110. It is to be understood that both light extraction element140 and semi-specular element 150 can be disposed anywhere within cavity130, for example both elements can be disposed adjacent front reflector110, adjacent back reflector 120, or at a position partway between thetwo reflectors. In some cases, light extraction element 140 andsemi-specular element 150 can be combined into a common optical element(not shown).

As shown in FIG. 1, the backlight 100 also includes one or more lightsources 160 disposed to emit light into the light recycling cavity 130.The one or more light sources 160 are configured to emit light into thelight recycling cavity 130 over a limited angular range. In theembodiment illustrated in FIG. 1, the light sources 160 are disposedproximate edge 132 of the cavity 130.

The backlight 100 can be any suitable size and shape. In someembodiments, the backlight 100 can have a length L and a width W of oneor more millimeters to several meters. Further, in some embodiments, twoor more backlights can be tiled together and controlled individually toprovide large zoned backlights.

As illustrated, backlight 100 includes an injector or reflector 165 thathelps to direct light from the one or more light sources 160 into thelight recycling cavity 130. Any suitable injector or reflector can beused with the backlight 100, e.g., wedges, parabolic reflectors, lenses,etc. See, e.g., the injectors described in U.S. Patent Application No.60/939,082, entitled COLLIMATING LIGHT INJECTORS FOR EDGE-LITBACKLIGHTS.

Although depicted as having one or more light sources 160 positionedalong one side or edge of the backlight 100, light sources can bepositioned along two, three, four, or more sides of the backlight 100.For example, for a rectangularly shaped backlight, one or more lightsources can be positioned along each of the four sides of the backlight.

The front reflector 110 can include any partially transmissive reflectoror reflectors, e.g., the partially transmissive reflectors described inco-owned U.S. Patent Application No. 60/939,079, entitled BACKLIGHT ANDDISPLAY SYSTEM USING SAME; and U.S. Patent Application No. 60/939,084,entitled THIN HOLLOW BACKLIGHTS WITH BENEFICIAL DESIGN CHARACTERISTICS.In some embodiments, the front reflector 110 can include one or morepolymeric multilayer reflective polarizing films as described, e.g., inU.S. Pat. No. 5,882,774 (Jonza et al.) entitled OPTICAL FILM; U.S. Pat.No. 6,905,220 (Wortman et al.) entitled BACKLIGHT SYSTEM WITH MULTILAYEROPTICAL FILM REFLECTOR; U.S. Pat. No. 6,210,785 (Weber et al.) entitledHEIGHT EFFICIENCY OPTICAL DEVICES; U.S. Pat. No. 6,783,349 (Neavin etal.) entitled APPARATUS FOR MAKING MULTILAYER OPTICAL FILMS; U.S. PatentPublication No. 2008/0002256 (Sasagawa et al.), entitled OPTICAL ARTICLEINCLUDING A BEADED LAYER; U.S. Pat. No. 6,673,425 (Hebrink et al.)entitled METHOD AND MATERIALS FOR PREVENTING WARPING IN OPTICAL FILMS;U.S. Patent Publication No. 2004/0219338 (Hebrink et al.) entitledMATERIALS, CONFIGURATIONS, AND METHODS FOR REDUCING WARPAGE IN OPTICALFILMS; and U.S. patent application Ser. No. 11/735,684 (Hebrink et al.)entitled OPTICAL ARTICLE AND METHOD OF MAKING.

In some embodiments, the partially transmissive front reflector 110 canprovide polarized light at the output surface. Suitable polarizing frontreflectors include, e.g., DBEF, APF, DRPF (all available from 3MCompany, St. Paul, Minn.), ARF, TOP, (both as described in the60/939,079 application), etc. In other embodiments, the partiallytransmissive front reflector can provide non-polarized light. Suitablenon-polarizing front reflectors include, e.g., perforated mirrors,microstructured films, etc. Further examples of non-polarizing films aredescribed, e.g., in U.S. Patent Application No. 60/939,084.

The front reflector 110 is partially transmissive and partiallyreflective for at least visible light. The partial transmissivity of thefront reflector 110 allows at least a portion of light within the cavity130 to be emitted through the output surface 135 of the cavity 130. Thefront reflector 110 can include any suitable film(s) and/or layer(s)that provide partial transmission and reflection to light incident uponthe front reflector 110 from inside the cavity 130.

In some embodiments, the front reflector 110 is operable to transmitpolarized light. In such embodiments, the front reflector 110 includesan on-axis average reflectivity of at least about 90% for visible lightpolarized in a first plane, and an on-axis average reflectivity of atleast about 5% but less than about 90% for visible light polarized in asecond plane parallel to the first plane. As used herein, the term“on-axis average reflectivity” refers to the average reflectivity oflight incident on a reflector in a direction that is substantiallynormal to such surface. Further, the term “total hemisphericalreflectivity” refers to the total reflectivity of a reflector for lightincident on the reflector from all directions within a hemispherecentered around a normal to the reflector. Those skilled in the artwould consider light polarized in the second plane to be in a useablepolarization state, i.e., such polarized light would pass through thelower absorbing polarizer of an LC panel (e.g., lower absorbingpolarizer 658 of FIG. 6) and be incident on the LC panel. Further, thoseskilled in the art would consider the first plane to be parallel withthe block axis and the second plane to be parallel to the pass axis ofthe polarizing front reflector 110.

Further, in some embodiments, it may be desirable that the averageon-axis transmission of the useable polarization state is several timesgreater than the transmission of non-useable polarization state toensure that the output from the cavity 130 is substantially the desiredpolarization state. This also helps to reduce the total loss of useablelight from the cavity. In some embodiments, the on-axis transmissivityof useable light to non-useable light is at least 10. In otherembodiments, the ratio of transmission of useable light to non-useablelight is at least 20.

In some embodiments, the front reflector 110 can include two or morefilms. For example, FIG. 4A is a schematic cross-section view of aportion of a front reflector 400. Reflector 400 includes a first film402 positioned proximate a second film 404. The films 402, 404 can bespaced apart or in contact with each other. Alternatively, the films402, 404 can be attached using any suitable technique. For example, thefilms 402, 404 can be laminated together using optional adhesive layer406. Any suitable adhesive can be used for layer 406, e.g., pressuresensitive adhesives (such as 3M Optically Clear Adhesives), andUV-curable adhesives (such as UVX-4856). In some embodiments, anadhesive layer 406 can be replaced with an index matching fluid, and thefilms 402, 404 can be held in contact using any suitable technique knownin the art.

Films 402, 404 can include any suitable films described herein in regardto the front reflector. Films 402, 404 can have similar opticalcharacteristics; alternatively, films 402, 404 can be differentconstructions that provide different optical characteristics. In oneexemplary embodiment, film 402 can include an asymmetric reflective filmas described herein having a pass axis in one plane, and film 404 caninclude a second asymmetric reflective film having a pass axis in asecond plane that is non-parallel to the pass axis of the first film402. This non-parallel relationship can form any suitable angle betweenthe two pass axis planes. In some embodiments, the pass axis planes canbe nearly orthogonal. Such a relationship would provide a high degree ofreflectivity in the pass axis for the front reflector 400.

Further, for example, film 402 may include an asymmetric reflectivefilm, and film 404 may include a prismatic brightness enhancing filmsuch as BEF. In some embodiments, the BEF may be oriented in relation tothe asymmetric reflective film such that the BEF collimates transmittedlight in a plane that is orthogonal to the collimating plane of theasymmetric film. Alternatively, in other embodiments, the BEF may beoriented such that the BEF collimates transmitted light in thecollimating plane of the asymmetric reflective film.

Although the front reflector 400 is depicted in FIG. 4A as including twofilms 402, 404, the front reflector 400 can include three or more films.For example, a three layer front reflector can be made using threelayers of reflective polarizer (such as DBEF or APF, both available from3M Company, St. Paul, Minn.). If the three layers are arranged such thatthe polarization axis of the second layer is at 45° relative to thepolarization axis of the first layer and the polarization axis of thethird layer is at 90° relative to the polarization axis of the firstlayer, the resulting front reflector will reflect approximately 75% ofthe normal incidence light. Other angles of rotation between the layerscould be used to achieve different levels of reflection. A birefringent(polarization rotating) layer or a scattering layer between tworeflective polarizers with nearly orthogonal pass axes can also createreflective films that have a controlled degree of reflectivity to beused as the front reflector.

The front reflectors of the present disclosure can also include opticalelements positioned in or on one or more surfaces of the reflector. Forexample, FIG. 4B is a schematic cross-section view of a portion ofanother embodiment of front reflector 410. The reflector 410 includes afilm 412 having a first major surface 414 and a second major surface416. The film 412 can include any suitable film(s) or layer(s) describedherein in regard to a front reflector. A plurality of optical elements418 are positioned on or in the first major surface 414. Althoughdepicted as positioned only on first major surface 414, optical elementscan be positioned on the second major surface 416 or on both first andsecond major surfaces 414, 416. Any suitable optical elements can bepositioned on or in the film 412, e.g., microspheres, prisms,cube-corners, lenses, lenticular elements, etc. The optical elements canbe refractive elements, diffractive elements, diffusive elements, etc.In this embodiment, the optical elements 418 can collimate light that istransmitted by film 412. In other embodiments, the optical elements 418can diffuse light either incident on the film 412 or exiting the film412, depending upon the positioning of the optical elements 412.

The optical elements 418 can be positioned on a major surface of thefilm 412 or at least partially embedded in the major surface of the film412. Further, the film 410 can be manufactured using any suitabletechnique, e.g., those techniques for manufacturing bead-coated ESRdescribed in U.S. Patent Application Nos. 60/939,079, entitled BACKLIGHTAND DISPLAY SYSTEM USING SAME, and 60/939,085, entitled RECYCLINGBACKLIGHTS WITH SEMI-SPECULAR COMPONENTS.

The optical elements 418 can also be positioned on a substrate that ispositioned proximate the film 410. For example, FIG. 4C is a schematiccross-section view of a portion of another embodiment of a frontreflector 420. The reflector 420 includes a film 422 and a gain diffuser424 positioned proximate the film 422. The film 422 can include anyfilm(s) and/or layer(s) described herein regarding front reflectors. Thegain diffuser 424 includes a substrate 426 having a first major surface428 and a second major surface 430, and a plurality of optical elements432 positioned on or in the second major surface 430 of the substrate426. Any suitable optical elements 432 can be used, e.g., opticalelements 418 of FIG. 4B. The substrate 426 can include any suitableoptically transmissive substrate.

For the embodiment illustrated in FIG. 4C, the first major surface 428of the gain diffuser 424 is positioned proximate the polarizing film422. The diffuser 424 can be positioned proximate film 422 such that itis spaced apart from the film 422, in contact with the film 422, orattached to the film 422. Any suitable technique can be used to attachthe diffuser 424 to the film 422, e.g., the use of optical adhesives.Any suitable gain diffuser can be used for diffuser 424. In someembodiments, the optical elements 432 can be positioned on the firstmajor surface 428 of the substrate 426 such that the elements 432 arebetween the substrate 426 and the polarizing film 422.

Returning to FIG. 1, the front reflector 110 can also be attached to asupporting layer. The support layer can include any suitable material ormaterials, e.g., polycarbonate, acrylic, PET, etc. In some embodiments,the front reflector 110 can be supported by a fiber reinforced opticalfilm as described, e.g., in U.S. Patent Publication No. 2006/0257678(Benson et al.), entitled FIBER REINFORCED OPTICAL FILMS; U.S. patentapplication Ser. No. 11/323,726 (Wright et al.), entitled REINFORCEDREFLECTIVE POLARIZER FILMS; and U.S. patent application Ser. No.11/322,324 (Ouderkirk et al.), entitled REINFORCED REFLECTIVE POLARIZERFILMS.

Further, the front reflector 110 can be attached to the support layerusing any suitable technique. In some embodiments, the front reflector110 can be adhered to the support layer using an optical adhesive. Inother embodiments, the front reflector 110 can be attached to an LCpanel of a display system (e.g., LC panel 650 of display system 600 ofFIG. 6). In such embodiments, the front reflector 110 can be attached toan absorbing polarizer and then attached to an LC panel, or,alternatively, the absorbing polarizer can first be attached to the LCpanel and then the front reflector 110 can be attached to the absorbingpolarizer. Further, in non-LCD systems, the front reflector 110 can beattached to a tinted front plate.

As mentioned herein, the front reflector 110 can include any suitablefilm(s) and/or layer(s) that provide a partially reflective andpartially transmissive front reflector. In some embodiments, the frontreflector 110 can include one or more fiber polarizing films asdescribed, e.g., in U.S. Patent Publication No. 2006/0193577 (Ouderkirket al.), entitled REFLECTIVE POLARIZERS CONTAINING POLYMER FIBERS; U.S.patent application Ser. No. 11/468,746 (Ouderkirk et al.), entitledMULTILAYER POLARIZING FIBERS AND POLARIZERS USING SAME; and U.S. patentapplication Ser. No. 11/468,740 (Bluem et al.), entitled POLYMER FIBERPOLARIZERS. Other exemplary films that can be used for the frontreflector 110 include cholesteric polarizing films, birefringentpile-of-plates films, birefringent polymer blends (e.g., DRPF, availablefrom 3M Company), and wire grid polarizers.

The films used for the front and back reflectors described herein can bemanufactured using any suitable technique. See, e.g., U.S. Pat. No.6,783,349 (Neavin et al.), entitled APPARATUS FOR MAKING MULTILAYEROPTICAL FILMS.

The front reflector 110 and the back reflector 120 can exhibit anysuitable value of R_(hemi). In general, the choice of R_(hemi) for ahollow backlight is influenced by the specific design criteria for agiven system. Primary design criteria often include display size (lengthand width), thickness, source lumens required to achieve a brightnesstarget for a given viewing angle, uniformity of brightness and/or color,and system robustness to variations in light sources, backlight opticalmaterials, or cavity dimensions. Additionally, the ability to spacelight sources far apart is an important system attribute, as itinfluences the minimum number of sources that are required, and thus thetotal cost of sources for the system. Lastly, the desired angularemission from the backlight can influence the choice of R_(hemi), sincethe angular emission characteristics achievable with polymericmultilayer optical films are dependent on this, with a larger range ofangular profiles possible with increasing R_(hemi).

One advantage of a lower R_(hemi) is a higher system efficiency.Generally, the less recycling that occurs, the lower the absorptivelosses from multiple reflections in the cavity. Any material in thebacklight cavity can absorb light, including the front and backreflectors, the side walls, support structure (e.g. posts), and thelight sources themselves. Light can escape through physical gaps in thecavity, or low level transmission through the edge reflectors or backreflector. Reducing the number of reflections reduces these losses,improves system efficiency, and reduces the required source lumens.

In some embodiments, the greater the ratio of cavity length to thickness(e.g., L/H), generally the greater the R_(hemi) required to transportthe light within the cavity. Thus for larger and/or thinner backlights,greater R_(hemi) values are generally required to achieve uniformity.

The further apart the desired spacing of the sources, generally thegreater the R_(hemi) that is desired so as to minimize non-uniformitybetween sources (the so-called “head-light effect”). Multiplereflections can help to fill in the darker areas between sources and, inthe case of RGB systems, reduce spokes of color by mixing the colors,resulting in a white appearance.

By varying the specularity of the light extraction element, we haveshown that for a given L/H, the R_(hemi) required to achieve uniformitycan be significantly reduced. This has the advantage of increasingsystem efficiency and reducing the source lumens required. However,reducing R_(hemi) reduces recycling, resulting in greater sensitivity tomanufacturing or component variations. System sensitivity to thefollowing variations increases with decreased recycling: dimensionalvariation including the varied thickness H, optical variations inreflectance or specularity of the front or back reflectors, opticalvariations in the reflectance or specularity of the light extractionelement, discontinuity of side reflectors, visibility of a supportstructure (e.g., posts), and color and brightness variations of thelight sources. In addition to increased sensitivity to the output of thelight sources, the system tolerance to in-service drift, differentialaging, or failure of the light sources is decreased with a lowerR_(hemi).

It is possible to design two systems having two R_(hemi) and the sameuniformity (for example, one having a shaped backplane or a gradientlight extraction element with a low R_(hemi), and the other a straightbackplane with a higher R_(hemi)) yet the sensitivity of the lowerR_(hemi) system can in some embodiments be greater than the higherR_(hemi) system. Here, manufacturability considerations may outweigh theincrease in system efficiency obtained by lowering R_(hemi). The choiceof R_(hemi) can depend on the specific design criteria for that system.

In the embodiment illustrated in FIG. 1, the front reflector 110 facesthe back reflector 120 to form cavity 130. The back reflector 120 ispreferably highly reflective. For example, the back reflector 120 canhave an on-axis average reflectivity for visible light emitted by thelight sources of at least 90%, 95%, 98%, 99%, or more for visible lightof any polarization. Such reflectivity values also can reduce the amountof loss in a highly recycling cavity. Further, such reflectivity valuesencompass all visible light reflected into a hemisphere, i.e., suchvalues include both specular and diffuse reflections.

The back reflector 120 can be a predominantly specular, diffuse, orcombination specular/diffuse reflector, whether spatially uniform orpatterned. In some embodiments, the back reflector 120 can be asemi-specular reflector as is further described herein. See also U.S.Patent Application No. 60/939,085, entitled RECYCLING BACKLIGHTS WITHSEMI-SPECULAR COMPONENTS; and U.S. patent application Ser. No.11/467,326 (Ma et al.), entitled BACKLIGHT SUITABLE FOR DISPLAY DEVICES.In some cases, the back reflector 120 can be made from a stiff metalsubstrate with a high reflectivity coating, or a high reflectivity filmlaminated to a supporting substrate. Suitable high reflectivitymaterials include Vikuiti™ Enhanced Specular Reflector (ESR) multilayerpolymeric film (available from 3M Company); a film made by laminating abarium sulfate-loaded polyethylene terephthalate film (2 mils thick) toVikuiti™ ESR film using a 0.4 mil thick isooctylacrylate acrylic acidpressure sensitive adhesive, the resulting laminate film referred toherein as “EDR II” film; E-60 series Lumirror™ polyester film availablefrom Toray Industries, Inc.; porous polytetrafluoroethylene (PTFE)films, such as those available from W. L. Gore & Associates, Inc.;Spectralon™ reflectance material available from Labsphere, Inc.; Miro™anodized aluminum films (including Miro™ 2 film) available from AlanodAluminum-Veredlung GmbH & Co.; MCPET high reflectivity foamed sheetingfrom Furukawa Electric Co., Ltd.; White Refstar™ films and MT filmsavailable from Mitsui Chemicals, Inc.; and 2×TIPS (i.e., a porouspolypropylene film having a high reflectivity and can be made usingthermally induced phase separation as described, e.g., in U.S. Pat. No.5,976,686 (Kaytor et al.). Two sheets of TIPS can be laminated togetherusing an optical adhesive to form a laminate.).

The back reflector 120 can be substantially flat and smooth, or it mayhave a structured surface associated with it to enhance light scatteringor mixing. Such a structured surface can be imparted (a) on the surfaceof the back reflector 120, or (b) on a transparent coating applied tothe surface. In the former case, a highly reflecting film may belaminated to a substrate in which a structured surface was previouslyformed, or a highly reflecting film may be laminated to a flat substrate(such as a thin metal sheet, as with Vikuiti™ Durable Enhanced SpecularReflector-Metal (DESR-M) reflector available from 3M Company) followedby forming the structured surface, such as with a stamping operation. Inthe latter case, a transparent film having a structured surface can belaminated to a flat reflective surface, or a transparent film can beapplied to the reflector and then afterwards a structured surfaceimparted to the top of the transparent film.

For those embodiments that include a direct-lit configuration (e.g.,backlight 300 of FIG. 3), the back reflector 120 can be a continuousunitary (and unbroken) layer on which the light source(s) are mounted,or it can be constructed discontinuously in separate pieces, ordiscontinuously insofar as it includes isolated apertures, through whichlight sources can protrude, in an otherwise continuous layer. Forexample, strips of reflective material can be applied to a substrate onwhich rows of LEDs are mounted, each strip having a width sufficient toextend from one row of LEDs to another and having a length dimensionsufficient to span between opposed borders of the backlight's outputarea.

In the embodiment illustrated in FIG. 1, the back reflector 120 is aplanar reflector, although backlight 100 can be a non-planar backreflector such that H varies across the cavity 130, as described in the61/030,767 application. In some embodiments, the back reflector 120 andpartially transmissive front reflector 110 can be curved in the samedirection (not shown), so that the distance H remains essentiallyconstant throughout the cavity 130. The space between the frontreflector and back reflector can be maintained using any suitabletechniques, e.g., a rigid plates, tensioning frames, and variousstructures in the cavity, including posts, walls, or protrusionsextending from the back reflector such as bumps or ridges.

The backlight 100 can also include one or more side reflectors 134located along at least a portion of the outer boundary of the backlight100 that are preferably lined or otherwise provided with highreflectivity vertical walls to reduce light loss and improve recyclingefficiency. The same reflective material used for the back reflector 120can be used to form these walls, or a different reflective material canbe used. In some embodiments, the side reflectors 134 and back reflector120 can be formed from a single sheet of material. One or both of theside reflectors and walls can be vertical, or, alternatively, the sidereflectors can be tilted, curved, or structured. Refractive structurescan be used on or adjacent to the side reflectors to achieve a desiredreflection profile. Wall material and inclination can be chosen toadjust the output light flux distribution.

The backlight 100 includes a light extraction element 140 that has agradient specularity, resulting in a gradient extraction of light. Thegradient extraction can be accomplished by any element that increases ordecreases locally the amount of light extraction. Since the partiallytransmissive front reflector generally has some degree of angularlyselective transmission, an extraction element that deviates additionallight into the angular range of higher transmission will increase thebrightness in that region. The extraction zone is generally towardnormal, but can be designed to be at oblique angles. The material thatis used for the extraction element can be specular, semispecular ordiffuse, translucent, transflective, refractive, diffractive,down-converting such as a phosphor, or the like. Refractive elements canbe prisms, lenslets, lenticulars, and the like. Extraction elements maybe applied by printing, casting, coating, embossing, etching, ablating,transfer (e.g., adhesive backed dots), lamination and the like.Extraction elements can also be made by local deviations in a reflectivesurface such as embossing, peening, corrugating, abrading, etching andthe like.

A gradient specularity can also be accomplished by a change in the lightre-directing properties of a diffusing coating, either locally orgradually across the surface area. This can be accomplished by, forexample, a change in thickness, composition, or surface properties.Perforations can also be used, for example a diffusing film having agradient of perforations. In addition, a reflective film having onespecular character (e.g. ESR) can be perforated and disposed over areflecting film having a different specular character (e.g. a diffusewhite reflector such as MCPET available from Furukawa, Japan), toprovide the gradient specularity.

As used herein, the term “gradient specularity” is meant a variation inthe light extraction capability (i.e. specularity) over the surfaceequivalent to the output area. In one embodiment, the gradientspecularity can be continuously variable and change smoothly, e.g. in amonotonic fashion across the surface of the light extraction element. Insome embodiments, the light extraction element can have a gradient inthe length direction, the width direction, or both the length and widthdirection. In some embodiments, the gradient can include one or morestep-changes, such as a circular region on a specular back reflectorused to make a bright center. In some embodiments, the gradient can bean array of discrete areas of extraction, such as an area of uniformspecularity that are different than the specularity in adjacent areas.In some embodiments, the gradient specularity can be a steppedspecularity, a discrete change in specularity, a continuous change inspecularity, a discontinuous change in specularity, a sequence ofmultiple regions of different specularity, or a combination of gradientspecularity.

The backlight 100 also includes one or more light sources 160 disposedto emit light into the light recycling cavity 130. In this embodiment,the light sources are positioned proximate edge 132 of backlight 100.The light sources 160 are shown schematically. In most cases, theselight sources 160 are compact light emitting diodes (LEDs). In thisregard, “LED” refers to a diode that emits light, whether visible,ultraviolet, or infrared. It includes incoherent encased or encapsulatedsemiconductor devices marketed as “LEDs”, whether of the conventional orsuper radiant variety. If the LED emits non-visible light such asultraviolet light, and in some cases where it emits visible light, it ispackaged to include a phosphor (or it may illuminate a remotely disposedphosphor) to convert short wavelength light to longer wavelength visiblelight, in some cases yielding a device that emits white light. An “LEDdie” is an LED in its most basic form, i.e., in the form of anindividual component or chip made by semiconductor processingprocedures. The component or chip can include electrical contactssuitable for application of power to energize the device. The individuallayers and other functional elements of the component or chip aretypically formed on the wafer scale, and the finished wafer can then bediced into individual piece parts to yield a multiplicity of LED dies.More discussion of packaged LEDs, including forward-emitting andside-emitting LEDs, is provided herein.

Multicolored light sources, whether or not used to create white light,can take many forms in a backlight, with different effects on color andbrightness uniformity of the backlight output area. In one approach,multiple LED dies (e.g., a red, a green, and a blue light emitting die)are all mounted in close proximity to each other on a lead frame orother substrate, and then encased together in a single encapsulantmaterial to form a single package, which may also include a single lenscomponent. Such a source can be controlled to emit any one of theindividual colors, or all colors simultaneously. In another approach,individually packaged LEDs, with only one LED die and one emitted colorper package, can be clustered together for a given recycling cavity, thecluster containing a combination of packaged LEDs emitting differentcolors such as blue/yellow or red/green/blue. In still another approach,such individually packaged multicolored LEDs can be positioned in one ormore lines, arrays, or other patterns.

If desired, other visible light emitters such as linear cold cathodefluorescent lamps (CCFLs) or hot cathode fluorescent lamps (HCFLs) canbe used instead of or in addition to discrete LED sources asillumination sources for the disclosed backlights. In addition, hybridsystems such as, for example, (CCFL/LED), including cool white and warmwhite, CCFL/HCFL, such as those that emit different spectra, may beused. Other suitable light sources include Xe CCFLs, flat fluorescentlamps, field emission sources, photonic lattice sources, vertical cavitysurface emitting lasers, external electrode fluorescent lamps, andorganic light emitting diodes. The combinations of light emitters mayvary widely, and include LEDs and CCFLs, and pluralities such as, forexample, multiple CCFLs, multiple CCFLs of different colors, and LEDsand CCFLs.

For example, in some applications it may be desirable to replace the rowof discrete light sources with a different light source such as a longcylindrical CCFL, or with a linear surface emitting light guide emittinglight along its length and coupled to a remote active component (such asan LED die or halogen bulb), and to do likewise with other rows ofsources. Examples of such linear surface emitting light guides aredisclosed in U.S. Pat. No. 5,845,038 (Lundin et al.) and U.S. Pat. No.6,367,941 (Lea et al.). Fiber-coupled laser diode and othersemiconductor emitters are also known, and in those cases the output endof the fiber optic waveguide can be considered to be a light source withrespect to its placement in the disclosed recycling cavities orotherwise behind the output area of the backlight. The same is also trueof other passive optical components having small emitting areas such aslenses, deflectors, narrow light guides, and the like that give offlight received from an active component such as a bulb or LED die. Oneexample of such a passive component is a molded encapsulant or lens of aside-emitting packaged LED.

Any suitable side-emitting LED can be used for one or more lightsources, e.g., Luxeon™ LEDs (available from Lumileds, San Jose, Calif.),or the LEDs described, e.g., in U.S. patent application Ser. No.11/381,324 (Leatherdale et al.), entitled LED PACKAGE WITH CONVERGINGOPTICAL ELEMENT; and U.S. patent application Ser. No. 11/381,293 (Lu etal.), entitled LED PACKAGE WITH WEDGE-SHAPED OPTICAL ELEMENT.

In some embodiments where the backlights are used in combination with adisplay panel (e.g., panel 650 of FIG. 6), the backlight 100continuously emits white light, and the LC panel is combined with acolor filter matrix to form groups of multicolored pixels (such asyellow/blue (YB) pixels, red/green/blue (RGB) pixels,red/green/blue/white (RGBW) pixels, red/yellow/green/blue (RYGB) pixels,red/yellow/green/cyan/blue (RYGCB) pixels, or the like) so that thedisplayed image is polychromatic. Alternatively, polychromatic imagescan be displayed using color sequential techniques, where, instead ofcontinuously back-illuminating the LC panel with white light andmodulating groups of multicolored pixels in the LC panel to producecolor, separate differently colored light sources within the backlight100 (selected, for example, from red, orange, amber, yellow, green,cyan, blue (including royal blue), and white in combinations such asthose mentioned above) are modulated such that the backlight flashes aspatially uniform colored light output (such as, for example, red, thengreen, then blue) in rapid repeating succession. This color-modulatedbacklight is then combined with a display module that has only one pixelarray (without any color filter matrix), the pixel array being modulatedsynchronously with the backlight to produce the whole gamut ofachievable colors (given the light sources used in the backlight) overthe entire pixel array, provided the modulation is fast enough to yieldtemporal color-mixing in the visual system of the observer. Examples ofcolor sequential displays, also known as field sequential displays, aredescribed in U.S. Pat. No. 5,337,068 (Stewart et al.) and U.S. Pat. No.6,762,743 (Yoshihara et al.). In some cases, it may be desirable toprovide only a monochrome display. In those cases the backlight 100 caninclude filters or specific sources that emit predominantly in onevisible wavelength or color.

In some embodiments, e.g., direct-lit backlights such as the embodimentillustrated in FIG. 3, the light sources may be positioned on the backreflector; alternatively, the light sources may be spaced apart from theback reflector. In other embodiments, the light sources may includelight sources that are positioned on or attached to the back reflector,e.g., as described in co-owned and copending U.S. patent applicationSer. Nos. 11/018,608; 11/018,605; 11/018,961; and 10/858,539.

The light sources 160 may be positioned in any suitable arrangement.Further, the light sources 160 can include light sources that emitdifferent wavelengths or colors of light. For example, the light sourcesmay include a first light source that emits a first wavelength ofillumination light, and a second light source that emits a secondwavelength of illumination light. The first wavelength may be the sameas or different from the second wavelength. The light sources 160 canalso include a third light source that emits a third wavelength oflight. See, e.g., U.S. Patent Application No. 60/939,083, entitled WHITELIGHT BACKLIGHTS AND THE LIKE WITH EFFICIENT UTILIZATION OF COLORED LEDSOURCES. In some embodiments, the various light sources 160 may producelight that, when mixed, provides white illumination light to a displaypanel or other device. In other embodiments, the light sources 160 mayeach produce white light.

Further, in some embodiments, light sources that at least partiallycollimate the emitted light may be preferred. Such light sources caninclude lenses, extractors, shaped encapsulants, or combinations thereofof optical elements to provide a desired output into the hollow lightrecycling cavity of the disclosed backlights. Exemplary extractors aredescribed, e.g., in U.S. Patent Publication Nos. 2007/0257266;2007/0257270; 2007/0258241; 2007/0258246; and U.S. Pat. No. 7,329,982.

Further, the backlights of the present disclosure can include injectionoptics that partially collimate or confine light initially injected intothe recycling cavity to propagation directions close to a transverseplane (the transverse plane being parallel to the output area of thebacklight), e.g., an injection beam having an average deviation anglefrom the transverse plane in a range from 0 to 45 degrees, or 0 to 30degrees, or 0 to 15 degrees.

In some embodiments of the present disclosure it may be preferred thatsome degree of diffusion be provided within the hollow light recyclingcavity. Such diffusion can provide more angular mixing of light withinthe cavity, thereby helping to spread the light within the cavity andprovide greater uniformity in the light directed out of the cavitythrough the output surface. In other words, the recycling optical cavitycontains a component that provides the cavity with a balance of specularand diffuse characteristics, the component having sufficient specularityto support significant lateral light transport or mixing within thecavity, but also having sufficient diffusivity to substantiallyhomogenize the angular distribution of steady state light propagationwithin the cavity, even when injecting light into the cavity only over anarrow range of propagation angles. Additionally, recycling within thecavity must result in a degree of randomization of reflected lightpolarization relative to the incident light polarization state. Thisallows for a mechanism by which unusable polarization light can beconverted by recycling into usable polarization light. The diffusion canbe provided by one or both of the front and back reflectors, the sidereflectors, or by one or more layers positioned between the front andback reflectors as is further described herein.

In some embodiments, the diffusion provided within the cavity caninclude semi-specular diffusion. As used herein, the term “semi-specularreflector” refers to a reflector that reflects substantially moreforward scattering than reverse scattering. Similarly, the term“semi-specular diffuser” refers to a diffuser that does not reverse thenormal component of the incident ray for a substantial majority of theincident light, i.e., the light is substantially transmitted in theforward (z) direction and scattered to some degree in the x and ydirections. In other words, semi-specular reflectors and diffusers(collectively referred to as semi-specular elements) direct the light ina substantially forward direction and thus are very different fromLambertian components which redirect light rays equally in alldirections. Semi-specular reflectors and diffusers can exhibitrelatively wide scattering angles; alternatively, such reflectors anddiffusers can exhibit only small amounts of light deflection outside thespecular direction. See, e.g., U.S. Patent Application No. 60/939,085,entitled RECYCLING BACKLIGHTS WITH SEMI-SPECULAR COMPONENTS. Anysuitable semi-specular material or materials can be used for the frontand back reflectors of the present disclosure.

Further, for example, the semi-specular back reflectors can include apartially transmitting specular reflector on a high reflectance diffuserreflector. Suitable partially transmitting specular reflectors includeany of the partially transmitting reflective films described herein,e.g., symmetric or asymmetric reflective films. Suitable highreflectance diffuse reflectors include EDR II film (available from 3M);porous polytetrafluoroethylene (PTFE) films, such as those availablefrom W. L. Gore & Associates, Inc.; Spectralon™ reflectance materialavailable from Labsphere, Inc.; MCPET high reflectivity foamed sheetingfrom Furukawa Electric Co., Ltd.; and White Refstar™ film available fromMitsui Chemicals, Inc.

In other embodiments, a semi-specular back reflector can include apartial Lambertian diffuser on a high reflectance specular reflector.Alternatively, a forward scattering diffuser on a high reflectancespecular reflector can provide a semi-specular back reflector.

The front reflector can be made semi-specular with constructions thatare similar to the back reflector. For example, a partial reflectingLambertian diffuser can be combined with a partial specular reflector.Alternatively, a forward scattering diffuser can be combined with apartial specular reflector. Further, the front reflector can include aforward scattering partial reflector. In other embodiments, any of theabove-described front reflectors can be combined to provide asemi-specular front reflector.

One or both of the front and back reflectors can be specular if adiffuser is placed somewhere in the cavity. One of the reflectors canalso be Lambertian, but in general this is not an optimum construction,particularly for edge-lit backlights. In this case, the other reflectorshould be semi-specular or specular. The forward scattering diffuserscan be any suitable diffuser and can be symmetric or asymmetric withrespect to both direction or polarization state.

Quantitatively, the degree of semi-specularity (specular vs. Lambertiancharacteristic of a given reflector or other component) can beeffectively characterized by comparing the fluxes of the forward- andback-scattered light components, referred to as F and B respectively.The forward and back-scattered fluxes can be obtained from theintegrated reflection intensities (or integrated transmissionintensities in the case of optically transmissive components) over allsolid angles. The degree of semi-specularity can then be characterizedby a “transport ratio” T, given by: T=(F−B)/(F+B).

T ranges from 0 to 1 as one moves from purely Lambertian to purelyspecular. For a pure specular reflector there is no back-scatter (B=0),and therefore T=F/F=1. For a pure Lambertian reflector, the forward- andback-scattered fluxes are the same (F=B), and thus T=0. Examples withexperimentally measured values are given below. The transport ratio forany real reflective or transmissive component is a function of incidenceangle. This is logical, because one would expect the amount offorward-scattered light, for example, to be different for anear-normally incident ray than for a grazing-incident ray.

In connection with a recycling cavity, one can define an “effectivecavity transport ratio”, i.e., the transport ratio experienced by agiven incident ray after a complete circuit or cycle of the recyclingcavity. This quantity may be of interest, particularly in cavities thatcontain at least one semi-specular component and at least one additionalscattering component (whether semi-specular or Lambertian). Sincetransport ratio is in general a function of incidence angle, one couldevaluate or specify the effective cavity transport ratio in terms of acharacteristic or average incidence angle of light injected into thecavity, e.g., the average power flux deviation angle of the lightsource(s). See, e.g., U.S. Patent Application No. 60/939,085 for furtherdiscussion of transport ratio.

Although not shown in FIG. 1, the backlight 100 (or display system 600of FIG. 6) can include a light sensor and feedback system to detect andcontrol one or both of the brightness and color of light from the lightsources 160. For example, a sensor can be located near individual lightsources 160 or clusters of sources to monitor output and providefeedback to control, maintain, or adjust a white point or colortemperature. It may be beneficial to locate one or more sensors along anedge or within the cavity 130 to sample the mixed light. In someinstances it may be beneficial to provide a sensor to detect ambientlight outside the display in the viewing environment, for example, theroom that the display is in. Control logic can be used to appropriatelyadjust the output of the light sources 160 based on ambient viewingconditions. Any suitable sensor or sensors can be used, e.g.,light-to-frequency or light-to-voltage sensors (available from TexasAdvanced Optoelectronic Solutions, Plano, Tex.). Additionally, thermalsensors can be used to monitor and control the output of light sources160. Any of these techniques can be used to adjust light output based onoperating conditions and compensation for component aging over time.Further, sensors can be used for dynamic contrast, vertical scanning orhorizontal zones, or field sequential systems to supply feedback signalsto the control system.

The output surface 135 of the backlight 100 can include any suitablearea in relation to the area of the cavity 130. For example, in someembodiments, the output surface 135 can be smaller in area than the area(L×W) of the cavity 130. This can be accomplished, e.g., using a frontreflector 110 that has a portion that is highly reflective, therebyreducing the effective area of the output surface 135. A reduced outputsurface area can increase the brightness provided by the backlight for agiven input flux from the light sources 160.

As mentioned herein, some backlights of the present disclosure caninclude one or more light sources positioned at one or more edges of thebacklight to form an edge-lit backlight. For example, FIG. 2 is aschematic cross-section view of another embodiment of an edge-litbacklight 200. The backlight 200 includes a partially transmissive frontreflector 210 and a back reflector 220 that form a hollow lightrecycling cavity 230 that includes an output surface 235. As is furtherdescribed herein, cavity 230 further includes at least one lightextraction element (in FIG. 2, elements 242, 244 and 246) having agradient specularity. The backlight 200 also includes a semi-specularelement 250 disposed within the hollow light recycling cavity 230, andat least two light sources 260, 270 disposed to emit light into thelight recycling cavity 230. The light sources 260, 270 are configured toemit light into the light recycling cavity 230 over a limited angularrange. All of the design considerations and possibilities describedherein regarding the front reflector 110, back reflector 120,semi-specular element 250, and one or more light sources 160 ofbacklight 100 of FIG. 1 apply equally to the front reflector 210, theback reflector 220, the light extraction elements 242, 244, 246, thesemi-specular element 250, and the light sources 260, 270 of thebacklight 200 of FIG. 2.

In the embodiment illustrated in FIG. 2, the one or more light sources260, 270 are disposed proximate first edge 232 and second edge 234 ofthe backlight 200, respectively. In other embodiments, light sources canbe disposed proximate any number of edges of the backlight. Asillustrated, backlight 200 includes a pair of injectors or reflectors265, 275 that helps to direct light from the one or more light sources260, 270 into the light recycling cavity 230. Any suitable injector orreflector can be used with the backlight 200, e.g., wedges, parabolicreflectors, etc. See, e.g., the injectors described in U.S. PatentApplication No. 60/939,082, entitled COLLIMATING LIGHT INJECTORS FOREDGE-LIT BACKLIGHTS.

In the illustrated embodiment, the light extraction element is separatedinto three segments (242, 244, 246) with second portions 248, 249 of theback reflector 220 between them. In some embodiments, as shown in FIG.1, light extractor element 140 is continuous across the length L of thebacklight 100. In some embodiments, as shown in FIG. 2, light extractionelement (242, 244, 246) is discontinuous across the length L of thebacklight 200, In some embodiments, not shown, the light extractionelement can be further divided into segments across the length and widthof a backlight, to provide the desired light flux output from the outputsurface.

Although backlight 200 of FIG. 2 is an edge-lit backlight having one ormore light sources positioned proximate edges of the backlight, otherembodiments can include light sources positioned to direct light intothe light recycling cavity within the area of cavity defined by theprojection of the output surface onto the back reflector, therebyforming a direct-lit backlight. For example, FIG. 3 is a schematiccross-section view of one embodiment of a direct-lit backlight 300. Thebacklight 300 includes a partially transmissive front reflector 310 anda back reflector 320 that form a hollow light recycling cavity 330having an output surface 335. The backlight 300 also includes at leastone semi-specular element (not shown) disposed within the hollow lightrecycling cavity 330, a light extraction element 340 having a gradientspecularity, and one or more light sources 360, 370 disposed to emitlight into the light recycling cavity 330. In the embodiment illustratedin FIG. 3, the light extraction element 340 is shown adjacent partiallytransmissive front reflector 310; however the light extraction element340 can be placed at other positions in cavity 330, as describedelsewhere. All of the design considerations and possibilities describedherein regarding the front reflector 110, the back reflector 120, thelight extraction element 140 having a gradient specularity, the at leastone semi-specular element 150, and the one or more light sources 160 ofthe backlight 100 of FIG. 1 apply equally to the front reflector 310,the back reflector 320, the light extraction element 340 having agradient specularity, the at least one semi-specular element, and theone or more light sources 360, 370 of the backlight 300 of FIG. 3.

In the embodiment illustrated in FIG. 3, the one or more light sources360, 370 are positioned within the light recycling cavity 330. In someembodiments, the light sources 360, 370 are configured to emit light ina substantially sideways direction such that the emitted light has anaverage flux deviation angle relative to a transverse plane defined bythe output surface 335 in a range of 0 to 40 degrees. In other words,the light sources 360, 370 can be configured to emit a substantialportion of light into the transport zone of the cavity 330. The one ormore light sources 360, 370 can be positioned in any suitable locationwithin the cavity 330.

In some direct-lit embodiments, generally vertical reflective sidesurfaces 332, 334 may actually be thin partitions that separate thebacklight from similar or identical neighboring backlights, where eachsuch backlight is actually a portion of a larger zoned backlight. Lightsources in the individual sub-backlights can be turned on or off in anydesired combination to provide patterns of illuminated and darkenedzones for the larger backlight. Such zoned backlighting can be useddynamically to improve contrast and save energy in some LCDapplications. The reflective partitions between zones may not extendcompletely to the front reflector, but may be separated therefrom by agap that is sized to minimize the visibility of zone boundaries (fromthe perspective of a viewer) while also minimizing zone-to-zonebleedthrough.

FIGS. 5 a-5 f schematic cross-section views of various embodiments ofedge-lit hollow backlights, according to one aspect of the disclosure.In FIGS. 5 a-5 f, backlight 500 includes a partially transmissive frontreflector 510 and a back reflector 520 that form a hollow lightrecycling cavity 530. Partially transmissive front reflector 510includes a major surface 512 that faces the back reflector 520. Backreflector 520 includes a major surface 522 that faces the partiallytransmissive front reflector 510. Backlight 530 further includes one ormore light sources (not shown), similar to those described elsewhere.

In FIG. 5 a, backlight 500 includes an optical element 540 disposedadjacent the major surface 512 of partially transmissive front reflector510. Optical element 540 is a semi-specular element that is combinedwith a light extraction element having gradient specularity.

In FIG. 5 b, backlight 500 includes a first optical element 540 disposedadjacent the major surface 512 of partially transmissive front reflector510, and a second optical element 541 disposed adjacent the majorsurface 522 of back reflector 520. First and second optical elements540, 541 are semi-specular elements that are combined with lightextraction elements having a gradient specularity. As depicted in FIG. 5b, the gradient specularity within each of the optical elements can bedifferent, or it can be the same.

In FIG. 5 c, backlight 500 includes an optical element 541 disposedadjacent the major surface 522 of back reflector 520. Optical element541 is a semi-specular element that is combined with a light extractionelement having gradient specularity.

In FIG. 5 d, backlight 500 includes an optical element 541 separated bya gap 543 from the major surface 522 of back reflector 520. Opticalelement 541 is a semi-specular element that is combined with a lightextraction element having gradient specularity.

In FIG. 5 e, backlight 500 includes a semi-specular element 544 disposedadjacent the major surface 512 of partially transmissive front reflector510, and a gradient light extractor comprising light extraction elements545 disposed in a gradient on major surface 522 of back reflector 520.

In FIG. 5 f, backlight 500 includes a semi-specular element 544 disposedadjacent the major surface 512 of partially transmissive front reflector510. Semi-specular element 544 has a major surface 546 facing backreflector 520, and a gradient light extractor comprising lightextraction elements 545 disposed in a gradient on major surface 546 ofsemi-specular element 544.

All of the design considerations and possibilities described hereinregarding the front reflector 110, the back reflector 120, the lightextraction element 140 having a gradient specularity, the at least onesemi-specular element 150, and the one or more light sources 160 of thebacklight 100 of FIG. 1 apply equally to the front reflector 510, theback reflector 520, the light extraction elements having a gradientspecularity, the at least one semi-specular element, and the one or morelight sources of the backlight 500 of FIGS. 5 a-5 f.

The backlights of the present disclosure can be implemented in anysuitable configuration or application. For example, the backlightsdescribed herein can be used with a display panel to form a displaysystem, e.g., an LC display or monitor. FIG. 6 is a schematiccross-section view of one embodiment of a display system 600. Thedisplay system 600 includes an LC panel 650 and an illumination assembly602 disposed to provide light to the LC panel 650. The LC panel 650typically includes a layer of LC 652 disposed between panel plates 654.The plates 654 are often formed of glass and may include electrodestructures and alignment layers on their inner surfaces for controllingthe orientation of the liquid crystals in the LC layer 652. Theseelectrode structures are commonly arranged so as to define LC panelpixels, i.e., areas of the LC layer where the orientation of the liquidcrystals can be controlled independently of adjacent areas. A colorfilter may also be included with one or more of the plates 652 forimposing color on the image displayed by the LC panel 650.

The LC panel 650 is positioned between an upper absorbing polarizer 656and a lower absorbing polarizer 658. In the illustrated embodiment, theupper and lower absorbing polarizers 656, 658 are located outside the LCpanel 650. The absorbing polarizers 656, 658 and the LC panel 650 incombination control the transmission of light from a backlight 610through the display system 600 to the viewer. For example, the absorbingpolarizers 656, 658 may be arranged with their transmission axesperpendicular to each other. In an unactivated state, a pixel of the LClayer 652 may not change the polarization of light passing therethrough.Accordingly, light that passes through the lower absorbing polarizer 658is absorbed by the upper absorbing polarizer 656. When the pixel isactivated, the polarization of the light passing therethrough is rotatedso that at least some of the light that is transmitted through the lowerabsorbing polarizer 658 is also transmitted through the upper absorbingpolarizer 656. Selective activation of the different pixels of the LClayer 652, for example, by a controller 604, results in the lightpassing out of the display system 600 at certain desired locations, thusforming an image seen by the viewer. The controller 604 may include, forexample, a computer or a television controller that receives anddisplays television images.

One or more optional layers 657 may be provided proximate the upperabsorbing polarizer 656, for example, to provide mechanical and/orenvironmental protection to the display surface. In one exemplaryembodiment, the layer 657 may include a hardcoat over the upperabsorbing polarizer 656.

It will be appreciated that some types of LC displays may operate in amanner different from that described above. For example, the absorbingpolarizers may be aligned parallel and the LC panel may rotate thepolarization of the light when in an unactivated state. Regardless, thebasic structure of such displays remains similar to that describedabove.

The illumination assembly 602 includes a backlight 610 and optionallyone or more light management films 640 positioned between the backlight610 and the LC panel 650. The backlight 610 can include any backlightdescribed herein, e.g., backlight 100 of FIG. 1.

An arrangement of light management films 640, which may also be referredto as a light management unit, is positioned between the backlight 610and the LC panel 650. The light management films 640 affect theillumination light propagating from the backlight 610. For example, thearrangement of light management films 640 may optionally include adiffuser 648. The diffuser 648 is used to diffuse the light receivedfrom the backlight 610.

The diffuser layer 648 may be any suitable diffuser film or plate. Forexample, the diffuser layer 648 can include any suitable diffusingmaterial or materials. In some embodiments, the diffuser layer 648 mayinclude a polymeric matrix of polymethyl methacrylate (PMMA) with avariety of dispersed phases that include glass, polystyrene beads, andCaCO₃ particles. Exemplary diffusers can include 3M™ Scotchcal™ DiffuserFilm, types 3635-30, 3635-70, and 3635-100, available from 3M Company,St. Paul, Minn.

The optional light management films 640 may also include a reflectivepolarizer 642. In some embodiments, the transmission axis of thereflective polarizer 642 can be aligned with the pass axis of the LCpanel 650. Any suitable type of reflective polarizer may be used for thereflective polarizer 642, e.g., multilayer optical film (MOF) reflectivepolarizers; diffusely reflective polarizing film (DRPF), such ascontinuous/disperse phase polarizers; wire grid reflective polarizers;or cholesteric reflective polarizers.

Both the MOF and continuous/disperse phase reflective polarizers rely onthe difference in refractive index between at least two materials,usually polymeric materials, to selectively reflect light of onepolarization state while transmitting light in an orthogonalpolarization state. Some examples of MOF reflective polarizers aredescribed in co-owned U.S. Pat. No. 5,882,774 (Jonza et al.).Commercially available examples of MOF reflective polarizers includeVikuiti™ DBEF-D200 and DBEF-D440 multilayer reflective polarizers thatinclude diffusive surfaces, available from 3M Company.

Examples of DRPF useful in connection with the present disclosureinclude continuous/disperse phase reflective polarizers as described,e.g., in co-owned U.S. Pat. No. 5,825,543 (Ouderkirk et al.), anddiffusely reflecting multilayer polarizers as described, e.g., inco-owned U.S. Pat. No. 5,867,316 (Carlson et al.). Other suitable typesof DRPF are described in U.S. Pat. No. 5,751,388 (Larson).

Some examples of wire grid polarizers useful in connection with thepresent disclosure include those described, e.g., in U.S. Pat. No.6,122,103 (Perkins et al.). Wire grid polarizers are commerciallyavailable from, inter alia, Moxtek Inc., Orem, Utah.

Some examples of cholesteric polarizers useful in connection with thepresent disclosure include those described, e.g., in U.S. Pat. No.5,793,456 (Broer et al.), and U.S. Patent Publication No. 2002/0159019(Pokorny et al.). Cholesteric polarizers are often provided along with aquarter wave retarding layer on the output side so that the lighttransmitted through the cholesteric polarizer is converted to linearlypolarized light.

In some embodiments, a polarization control layer 644 may be providedbetween the diffuser plate 648 and the reflective polarizer 642.Examples of polarization control layers 644 include a quarter waveretarding layer and a polarization rotating layer such as a liquidcrystal polarization rotating layer. The polarization control layer 644may be used to change the polarization of light that is reflected fromthe reflective polarizer 642 so that an increased fraction of therecycled light is transmitted through the reflective polarizer 642.

The optional arrangement of light management films 640 may also includeone or more brightness enhancing layers. A brightness enhancing layer isone that includes a surface structure that redirects off-axis light in adirection closer to the axis of the display. This increases the amountof light propagating on-axis through the LC layer 652, thus increasingthe brightness of the image seen by the viewer. One example of abrightness enhancing layer is a prismatic brightness enhancing layer,which has a number of prismatic ridges that redirect the illuminationlight through refraction and reflection. Examples of prismaticbrightness enhancing layers that may be used in the display system 600include the Vikuiti™ BEF II and BEF III family of prismatic filmsavailable from 3M Company, including BEF II 90/24, BEF II 90/50, BEFIIIM 90/50, and BEF IIIT. In some embodiments, a polarization preservingrefractive structure or structures can be utilized. Many types ofrefractive brightness enhancement films are highly birefringent and candepolarize the light emitted from the reflective polarizer. Substratessuch as polycarbonate can be made to be sufficiently isotropic so as notto depolarize.

Brightness enhancement may also be provided by some of the embodimentsof front reflectors as is further described herein.

The exemplary embodiment illustrated in FIG. 6 shows a first brightnessenhancing layer 646 a disposed between the reflective polarizer 642 andthe LC panel 650. A prismatic brightness enhancing layer typicallyprovides optical gain in one dimension. An optional second brightnessenhancing layer 646 b may also be included in the arrangement of lightmanagement films 640, having its prismatic structure orientedorthogonally to the prismatic structure of the first brightnessenhancing layer 646 a. Such a configuration provides an increase in theoptical gain of the display system 600 in two dimensions. In otherexemplary embodiments, the brightness enhancing layers 646 a, 646 b maybe positioned between the backlight 610 and the reflective polarizer642.

The different layers in the optional light management films 640 may befree standing. In other embodiments, two or more of the layers in thelight management films 640 may be laminated together, for example asdiscussed in co-owned U.S. patent application Ser. No. 10/966,610 (Ko etal.). In other exemplary embodiments, the optional light managementfilms 640 may include two subassemblies separated by a gap, for example,as described in co-owned U.S. patent application Ser. No. 10/965,937(Gehlsen et al.).

In one aspect, one or both of the front and back reflectors of thebacklights of the present disclosure can be positioned or shaped toprovide a desired output light flux distribution. In another aspect, oneor more light extraction elements can be positioned within the cavity toprovide a desired output light flux distribution. Any suitable techniquecan be used to determine what shape or position the reflectors or lightextraction elements should take to provide the desired distribution. Forexample, a hollow light recycling cavity having an output surface can beformed. The cavity can include a partially transmissive front reflectorand a planar back reflector. One or more light sources can be positionedto emit light into the light recycling cavity over a limited angularrange. A desired output light flux distribution can be selected. A firstoutput light flux distribution can be measured and compared to thedesired output light flux distribution. In one aspect, one or both ofthe front and back reflectors can then be shaped or positioned toprovide the desired output light flux distribution. In another aspect, alight extraction element can be positioned within the cavity to providethe desired output light flux distribution. In yet another aspect, oneor both of the front and back reflectors can then be shaped orpositioned, and a extraction element can be positioned within the cavityto provide the desired output light flux distribution. A second outputlight flux distribution can be measured and compared to the desiredoutput light flux distribution. Further shaping, forming, or positioningof one or both of the front and back reflectors, or changing thegradient specularity or position of the light extraction element, canthen be performed to provide the desired output light flux distribution.Any or all of the above-mentioned techniques can also be performed usingany suitable computer modeling technique known in the art.

Unless otherwise indicated, references to “backlights” are also intendedto apply to other extended area lighting devices that provide nominallyuniform illumination in their intended application. Such other devicesmay provide either polarized or unpolarized outputs. Examples includelight boxes, signs, channel letters, and general illumination devicesdesigned for indoor (e.g. home or office) or outdoor use, sometimesreferred to as “luminaires.” Note also that edge-lit devices can beconfigured to emit light out of both opposed major surfaces—i.e., bothout of the “front reflector” and “back reflector” referred to above—inwhich case both the front and back reflectors are partiallytransmissive. Such a device can illuminate two independent LCD panels orother graphic members placed on opposite sides of the backlight. In thatcase the front and back reflectors may be of the same or similarconstruction. Such two-sided backlights can be used, e.g., fordouble-sided signs, cell phones, etc. In some embodiments, a two-sidedbacklight can include a reflective member positioned within the cavityto direct light out of one or both major surfaces of the backlight. Thisreflective member can be fully reflective, partially transmissive, orcan have a combination of reflective and transmissive properties.Further, one or both major surfaces of the reflective member can beshaped as is described herein. Any suitable reflector can be used forthe reflective member.

The term “LED” refers to a diode that emits light, whether visible,ultraviolet, or infrared. It includes incoherent encased or encapsulatedsemiconductor devices marketed as “LEDs,” whether of the conventional orsuper radiant variety. If the LED emits non-visible light such asultraviolet light, and in some cases where it emits visible light, it ispackaged to include a phosphor (or it may illuminate a remotely disposedphosphor) to convert short wavelength light to longer wavelength visiblelight, in some cases yielding a device that emits white light.

Phosphors can be mixtures of fluorescent material in a binder. Thefluorescent material could be inorganic particles, organic particles, ororganic molecules or a combination thereof. Suitable inorganic particlesinclude doped garnets (such as YAG:Ce and (Y,Gd)AG:Ce), aluminates (suchas Sr₂Al₁₄O₂₅:Eu, and BAM:Eu), silicates (such as SrBaSiO:Eu), sulfides(such as ZnS:Ag, CaS:Eu, and SrGa₂S₄:Eu), oxy-sulfides, oxy-nitrides,phosphates, borates, and tungstates (such as CaWO₄). These materials maybe in the form of conventional phosphor powders or nanoparticle phosphorpowders. Another class of suitable inorganic particles is the so-calledquantum dot phosphors made of semiconductor nanoparticles including Si,Ge, CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, PbS, PbSe, PbTe, InN, InP, InAs,AlN, AlP, AlAs, GaN, GaP, GaAs and combinations thereof. Generally, thesurface of quantum dot will be at least partially coated with an organicmolecule to prevent agglomeration and increase compatibility with thebinder. In some cases the semiconductor quantum dot may be made up ofseveral layers of different materials in a core-shell construction.Suitable organic molecules include fluorescent dyes such as those listedin U.S. Pat. No. 6,600,175. Preferred fluorescent materials are thosethat exhibit good durability and stable optical properties. The phosphorlayer may consist of a blend of different types of phosphors in a singlelayer or a series of layers, each containing one or more types ofphosphors. The inorganic phosphor particles in the phosphor layer mayvary in size (diameter) and they may be segregated such that the averageparticle size is not uniform across the cross-section of the layer. Forexample, the larger particles may tend to be on one side of the filmwhile the smaller particles may tend to be located on the other side.This segregation could be accomplished by allowing the particles tosettle before the binder is cured. Other suitable phosphors include thinfilm phosphors, e.g., Lumiramic™ phosphor technology, available fromLumileds, San Jose, Calif.

An “LED die” is an LED in its most basic form, i.e., in the form of anindividual component or chip made by semiconductor processingprocedures. The component or chip can include electrical contactssuitable for application of power to energize the device. The individuallayers and other functional elements of the component or chip aretypically formed on the wafer scale, and the finished wafer can then bediced into individual piece parts to yield a multiplicity of LED dies.An LED may also include a cup-shaped reflector or other reflectivesubstrate, encapsulating material formed into a simple dome-shaped lensor any other known shape or structure, extractor(s), and other packagingelements, which elements may be used to produce a forward-emitting,side-emitting, or other desired light output distribution.

Unless otherwise indicated, references to LEDs are also intended toapply to other sources capable of emitting bright light, whether coloredor white, and whether polarized or unpolarized, in a small emittingarea. Examples include semiconductor laser devices, and sources thatutilize solid state laser pumping, solid state light sources thatincorporate photonic crystals, e.g., Phlatlight™ light sources,available from Luminus Devices, Inc. Billerica, Mass., and sources thatincorporate quantum well down-converting elements such as quantum dotsor quantum wells (see, e.g., U.S. Patent Application No. 60/978,304; andU.S. Patent Publication No. 2006/0124918).

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forth inthe foregoing specification and attached claims are approximations thatcan vary depending upon the desired properties sought to be obtained bythose skilled in the art utilizing the teachings disclosed herein.

EXAMPLES

Backlights were modeled using LightTools® 6.0.0 (available from OpticalResearch Associates, Pasadena, Calif.) and a Virtual Backlightsimulation tool, such as described in PCT Patent Application No.US2008/068953 entitled VIRTUAL BACKLIGHT FRAMEWORK.

Example 1 Edgelit Backlight

A 32″ (81.3 cm) diagonal 16:9 backlight was modeled. The backlight wasdesigned similar to FIG. 1 with the following dimensions: length L was398.5 mm, height H was 15.0 mm, the parabolic reflector (element 165 inFIG. 1) measured 21 mm deep and 14 mm wide (depth is the distance to theleft, and width is opening along edge 132), and the light source 160measured 2.6 mm. The side reflector (element 134 in FIG. 1) was tiltedabout 3.8° from the vertical (the end nearest element 110 is closer tothe light source). A beaded gain diffuser (prepared according to theprocedure described in Example F of copending U.S. Patent ApplicationNo. 60/939,085, entitled RECYCLING BACKLIGHTS WITH SEMI-SPECULARCOMPONENTS), was used as a semi-specular element (element 150 in FIG.1). All other interior surfaces of the light engine and cavity werelined with ESR, available from 3M company, unless otherwise specified.Simulations without light extraction elements were run, using twodifferent partially transmissive front reflectors (element 110 in FIG.1). The first partially transmissive front reflector (designated as ARF)was an Asymmetric Reflecting Film (ARF) having 32% transmission in thepass-direction, and a hemispherical reflectivity of 83%. The secondpartially transmissive front reflector (designated as APF) was anAdvanced Polarizing Film having a hemispherical reflectivity of 51%.

A two segment light extraction element was placed in the cavity,adjacent the back reflector as depicted in FIG. 1. Each of the twosegments of light extraction element included white dots with varieddensity and lambertian scattering property applied on the backreflector. The white dot patterns used in the simulations werecalculated using the “Bezier Placement Option” in LightTools® 6.0.0 fromOptical Research Associate. The first segment of light extractors was 50mm in length, and was next to the light source (starting at element 132in FIG. 1 and extending 50 mm). The second segment of light extractorswas 200 mm in length, and covered the back reflector from the midpointto the side reflector (element 134 in FIG. 1). Each of the white dotswas a Lambertian reflector having a reflectivity of 0.98. The white dotswere placed in the array for each segment as indicated Table 1. Theplacement along the length direction L was determined by the Bezierplacement option using the parameters indicated with an asterisk (*).

TABLE 1 First Segment Second Segment Number of dots along L direction*40 300 Number of dots along W direction 10 15 Dot separation along Wdirection 0.58 mm 0.38 mm Position* 0.0 7.908 Cell* 22.62 0.0 Weight*0.5 0.5 Dots Radius* 0.15 mm 0.15 mm

Simulations with the light extraction elements were run, using twodifferent partially transmissive front reflectors (element 110 in FIG.1). The first partially transmissive front reflector was an AsymmetricReflecting Film (ARF) having 32% transmission in the pass-direction, anda hemispherical reflectivity of 83% (designated ARF-dots). The secondpartially transmissive front reflector was an Advanced Polarizing Filmhaving a hemispherical reflectivity of 51% (designated APF-dots). FIG. 7is a graph of the modeled output light flux versus position from thelight source, including results from the ARF, ARF-dots, APF, andAPF-dots simulations.

The simulation results demonstrate that a light extraction elementhaving white print dots with lambertian scattering can improve displayuniformity. The display uniformity was improved for partiallytransmissive front reflectors having both low and high hemisphericalreflectivity. The efficiency of the backlight was also improved by theuse of a partially transmissive front reflector having a lowerhemispherical reflectivity, in part due to less recycling (and thereforeless absorptive losses) inside the hollow cavity.

All references and publications cited herein are expressly incorporatedherein by reference in their entirety into this disclosure, except tothe extent they may directly contradict this disclosure. Illustrativeembodiments of this disclosure are discussed and reference has been madeto possible variations within the scope of this disclosure. These andother variations and modifications in the disclosure will be apparent tothose skilled in the art without departing from the scope of thedisclosure, and it should be understood that this disclosure is notlimited to the illustrative embodiments set forth herein. Accordingly,the disclosure is to be limited only by the claims provided below.

What is claimed is:
 1. A backlight, comprising: a partially transmissivefront reflector and a back reflector that form a hollow light recyclingcavity comprising an output surface; a semi-specular element disposedwithin the hollow light recycling cavity; a light extraction elementdisposed within the hollow light recycling cavity, wherein the lightextraction element has a gradient specularity; at least one light sourcedisposed to inject light into the hollow light recycling cavity, whereinthe output surface defines a transverse plane, and the light sourceinjects light into the hollow light recycling cavity with an averageflux deviation angle relative to the transverse plane in a range from 0to 40 degrees.
 2. The backlight of claim 1, wherein the light extractionelement comprises a gradient diffuser.
 3. The backlight of claim 2,wherein the gradient diffuser comprises beads distributed on a surfaceof the light extraction element.
 4. The backlight of claim 2, whereinthe gradient diffuser comprises particulates dispersed within the lightextraction element.
 5. The backlight of claim 1, wherein the lightextraction element comprises an array of extractor features.
 6. Thebacklight of claim 5, wherein the array of extractor features comprisesmicrostructures, textures, or bumps.
 7. The backlight of claim 5,wherein the array of extractor features comprise embossed, ablated, orcoated extraction features.
 8. The backlight of claim 1, wherein thepartially transmissive front reflector comprises a hemisphericalreflectivity for unpolarized visible light of R_(hemi) ^(f), and theback reflector comprises a hemispherical reflectivity of unpolarizedvisible light of R_(hemi) ^(b) and wherein R_(hemi) ^(f)*R_(hemi) ^(b)is at least 0.45.
 9. The backlight of claim 8, wherein R_(hemi) ^(b) isgreater than about 0.95.
 10. The backlight of claim 8, wherein R_(hemi)^(f) is greater than about 0.50.
 11. The backlight of claim 1, whereinthe semi-specular element comprises a transport ratio greater than 15%at a 15 degree incidence angle and less than 95% at a 45 degreeincidence angle.
 12. The backlight of claim 1, wherein the semi-specularelement comprises the light extraction element.
 13. The backlight ofclaim 12, wherein the semi-specular element is disposed on a majorsurface of the partially transmissive front reflector that faces theback reflector.
 14. The backlight of claim 1, wherein the partiallytransmissive front reflector comprises an advanced polarizing film(APF), a dual brightness enhancement film (DBEF), a bulk diffuser, asurface diffuser, or a combination thereof.
 15. The backlight of claim1, wherein the back reflector comprises a metal, a metalized film, anenhanced specularly reflector (ESR) film, or a combination thereof. 16.The backlight of claim 1, wherein the at least one light sourcecomprises a light emitting diode (LED), and LED array, a collimated LED,or a combination thereof.
 17. The backlight of claim 1, furthercomprising at least one light sensor capable of providing a signal to afeedback control.
 18. The backlight of claim 1, further comprising: afirst zone comprising a first light sensor; and a second zone comprisinga second light sensor, wherein each of the first and second lightsensors are capable of providing a signal to control a first lightoutput in the first zone and a second light output in the second zone.19. A sign comprising the backlight of claim
 1. 20. A luminairecomprising the backlight of claim
 1. 21. A display system, comprising: adisplay panel; and a backlight disposed to provide light to the displaypanel, the backlight comprising: a partially transmissive frontreflector and a back reflector that form a hollow light recycling cavitycomprising an output surface; a semi-specular element disposed withinthe hollow light recycling cavity; a light extraction element disposedwithin the hollow light recycling cavity, wherein the light extractionelement has a gradient specularity; and at least one light sourcedisposed to inject light into the hollow light recycling cavity, whereinthe output surface defines a transverse plane, and the light sourceinjects light into the hollow light recycling cavity with an averageflux deviation angle relative to the transverse plane in a range from 0to 40 degrees.